Method of multi-coloring anodized aluminum



July 30, 1963 A. CYBRIWSKY ETAL. 3,099,610

METHOD OF MULTI-COLORING ANODIZED ALUMINUM Filed July 29, 1957 BRIGHTEN BRIGHTEN ANODIZE ETCH l RINSE RINSE ETCH ANODIZE i RINSE RINSE l COLOR COLOR L RINSE RINSE 1 i SEAL SEAL FIG. I FIG. 2

INVENTOR. ALEXANDER CYBRIVISKY GERALD L.HAGER BY kn LMW THEIR ATTORNEY United States Patent METHOD OF MULTI-CQLORING ANODIZED ALUMENUM Alexander Cyhriwsky and Gerald L. Hager, Louisville, Ky, assignors to Reynolds Metals Company, Richmend, Va., a corporation of Delaware Filed July 29, 1957, Ser. No. 674,833 1 Claim. (Cl. 204-35) This invention relates to a novel method of coloring a metal surface with two or more shades of one basic color. The invention, being particularly suited for use with aluminum, is herein described in connection therewith although not limited thereto. The terms aluminum and aluminous, as used herein, include pure aluminum, commercial aluminum containing the usual impurities and aluminum base alloys.

The important objects of the invention are: to provide a rapid process for dyeing a metal article having a microporous surface to obtain dye color contrast in one dyeing step; to avoid a separate dyeing treatment for each shade; to eliminate the usual masking operations occurring between successive dyeing treatments; to provide a coloring process wherein all materials applied to the article to be colored are water soluble or in aqueous media thereby obviating the use of organic liquids and extra drying steps; and to provide a process for producing patterns of different shades which may accurately delineate, if required, very small printing matter and intricate designs which may include sharp boundaries and fine lines.

We have found that, when an al-uminous surface is either (1) selectively etched to modify the surface smoothness and character of selected areas of the surface and anodized or (2) anodized and then selectively etched to modify the thickness and character of the anodic film at the selected areas of the surface, the natural ability of that anodized surface to absorb is changed in the selected areas. Accordingly, our invention resides in a method of treating an aluminous surface to obtain color contrast by providing a porous adsorbent anodized surface having different surface sections or surface areas of different adsorption characteristics and then coloring that surface with a given dye solution. When this is done, the surface areas of a given adsorptive ability will adsorb the coloring solution to a corresponding degree while the other surfaces of different adsorptive ability will adsorb the same dye solution to a correspondingly different degree. As a result a two-tone color effect can be readily obtained.

The practice of our invention is illustrated in the accompanying drawing, wherein:

FIG. 1 is a flow diagram of one method for practicing the invention; and

FIG. 2 is a flow diagram of a modified method.

In either case, the light reflective properties and the dye receptivity of the chemically treated areas are observably different than in the untreated areas, and this results in the production of a plurality of shades after the oxide film is dyed.

The practice of our invention is illustrated in the accompanying drawing, wherein:

FIG. 1 FLOW DIAGRAM In the process of FIG. 1, the aluminum surface is preliminarily treated preferably by a brightening operation and thereafter anodized, selectively etched, dyed and sealed.

Brightening The gloss or finish desired in the final colored product is controlled to a major extent by one or more of various brightening or polishing treatments that may be per- 3,099,610 Patented July so, 1963 ice formed on the metal article prior to the above named essential steps of the process. The finishing treatments include such mechanical operations as degreasing with petroleum solvents, grinding and polishing with abrasives, scratch-brushing with stainless steel or nickel wire wheels, sand blasting, embossing and others. Chemical and/or electrolytic treatments may also precede the initial etching or oxide coating treatment of this invention for brightening and polishing the metal.

As employed herein, the term brightening typifies any one or more of these preliminary treatments which are intended to provide surface characteristics other than the color of the oxide coating. For example, prior to the etching or oxidizing step (whichever comes first in the flow diagrams), an aluminum casting may be sand blasted, then ground to remove surface variations, and then subjected to roughing preparatory to polishing. Subsequently, polishing may be effected by rubbing the article with fine-particle abrasives, or by a chemical brightening procedure such as immersing the article for a few minutes in a conventional hot concentrated solution of phosphoric and sulfuric acids.

On the other hand, the article may consist of a rod or sheet of cold-rolled aluminum having an extremely smooth surface requiring only the removal of a light coating of oxide which ordinarily forms with exposure to the air. In the latter case, a brief immersion in a sulfuric acid bath followed by a rinse exposes the glossy surface of the metal and provides the desired brightening as the term is employed herein. An oxide coating as referred to hereinbelow is intended to relate only to those coatings produced artificially, and not to the very thin natural film of oxide which normally forms on aluminum surfaces in contact with the atmosphere.

Anodizing As the present invention is concerned essentially with the contrasting coloring, obtained in a single coloring operation, of separate areas of an oxide coating integrally formed on the parent or base metal, an essential element of the invention is the provision of this coating. Aluminum, for example, may be subjected to various chemical as well as chemical-electrolytic processes to produce a thin oxide coating thereon bonded integrally with the unreacted sub-layer of metal. Preferably it is anodized in a suitable electrolytic solution of sulfuric acid or chromic acid or oxalic acid.

A dye-susceptible oxide coating may also be formed by a chemical method, for example, wherein aluminum is immersed in a hot solution of 5% sodium carbonate containing a small amount of sodium chromate and heated to around 200 F. for 3 to 5 minutes. A proprietary process, known as anodizing, developed by the Colonial Alloys Co. of Philadelphia, is said to provide coatings similar to those produced by the sulfuric acid anodizing process.

An example of a conventional anodizing process for producing the oxide coating, an aluminum article may be placed in a sulfuric acid bath of 15% concentration with an electrolyzing current maintained across the electrodes at 15 to 20 volts and at an amperage density of approximately 12 to 30 amperes per square foot during a period of 10 to 60 minutes to yield a coating of from 0.0001 to 0.0007 of an inch in thickness. An anodic film of from 0.0001 to 0.0002 of an inch in thickness is entirely satisfactory. The micro-porous coating resulting from such an electrolysis reproduces to a substantial degree the original finish of the surface of the parent uncoated metal.

Generally speaking, a highly polished metal provides a better base upon which to practice the present method than a poorly polished metal since it permits a greater range in degree of etching by which to obtain progressively deeper shades of :a single color, while at the same time maintaining a higher level in the brightness or finish of the various shaded areas.

Selective Etching The etching or equivalent chemical treatment of selected areas in the anodic film has the effect of reducing its thickness and forming new micro-pores and enlarging and deepening existing pores. The selectively etched areas are thus capable of receiving and retaining a greater amount of the dye material than the untreated areas and, consequently, dye to a relatively deeper shade.

In general, the degree of etching may be controlled by the length of time the etching composition is left on the surface of the coating, and the strength of the composition. As it is possible to use etching compositions which are too reactive, the composition should be formulated in consideration of the time needed for progressively applying the composition to the work or ware and also in consideration of the time needed to transfer pieces through an etch-removing station so that an area designated for being colored to one shade only, will receive uniform etching.

As one important aspect of the invention, the basic process may be carried out with several successive etching applications, applied, e.g., by tandemly arranged printing rolls, without any intervening treatments of a dilferent nature, to obtain areas which are etched in different degrees. The etching step may comprise a plurality of applications of the etching composition, varying either, or both, in strength or period of deposition on the work, the etching action of which may be termin-ated by the single rinsing step which follows the etching step as illustrated. All areas being rinsed free of the etching composition, the ware may then pass to a station wherein it receives a common dyeing treatment. In this manner, an article may be dyed to many shades of the same color. In a Well-planned process involving multiple etching applications, the etching periods between successive applications, the strengths of the etching compositions, and the rinsing operation, are carefully correlated to provide the color pattern desired.

Although the active etching component of an etching composition may vary in concentration from A to 25% or more, compositions are preferred in which the concentration of the active component is sufliciently low, e.g., from A1. to about 6%, to permit close control of the process in modifying the metal or metal oxide surface to that degree required for obtaining the shades desired of a single color by a single dye treatment. Moreover, thick gum-my compositions appear to be better adapted for the maintenance of accurate boundary definition throughout the process with a minimium of overrunning and splattering. The need for compositions which may be applied in a rapid manner to accurately maintain a desired pattern on the ware until stripped therefrom becomes greater as the rate of coloring the ware is increased.

Among those compounds which are preferred as etching agents are hydrochloric and hydrofluoric acids, or mixtures thereof. Preferably, compositions of these agents contain thickening materials, such as gum arabic, ethyl cellulose, methyl cellulose, hydroxy ethyl cellulose, gelatin and tragacanth. Nitric acid provides smooth etching on both the metal and the oxide coating and is normally accompanied by less deterioration of gloss of the ware than many etching materials, but, in general, it may not be used with organic thickening agents because of the destructive effect thereon. Solutions of alkalies, e.g., sodium hydroxide and potassium hydroxide, may also provide the basis of etching compositions her use on the bare metal.

By adjustment of viscosity, etching compositions may be formulated for use as etching inks to be written directly on the ware with a glass pen, or as stamping inks for the production of printed patterns. For commercial production, etched designs may be produced on oxidecoated or bare-metal sheets by cutting patterns with the designs out of any absorbent and chemically-resistant sheet material, such as paper, which may be attached peripherally on a roller whereby the etching composition can be transferred to the metal sheets by rolling. Highly viscous compositions may also be applied, for example, by rolling contact of the high sur aces of an embossed rubber roll with the ware.

In general, it is desired to stop the action of the etching composition promptly at the end of the etching period. This is done by rinsing with water, preferably directed at considerable velocity against the etched surface, such as in the form of a jet or spray issuing through suitable nozzles, in order to assure that the pores in the oxide surface or the bare metal are cleansed 0f the etching composition. This precaution is normally observed to obtain good brilliance and fastness of the subsequently applied dye.

Dyeing In the dyeing step of the process herein described, dyeing procedures are followed which are conventional for coloring anodized or chemically produced oxide coatings. In carrying out such procedures, various dye materials capable of entering the micro-porous structure of the coatings may be used, including organic dyes, such as the diazo dyes and the phthalocy-anine dyes, and inorganic materials sueh as ferric ammonium oxalate, Zinc acetate followed by potassium dichrornate, and potassium ferrocyanide, followed by ferric chloride.

Sealing Permanently fixing the color within the micro-porous structure of the coating ordinarily involves dipping the work pieces into a bath capable of rendering a quick seal, for example, into a hot nickel or cobalt acetate bath for a minute or two to render the dye leach-resistant, and finally, dipping the work into a hot water maintained at boiling temperature or closely thereunder to provide permanent sealing.

EXAMPLE I A sheet of aluminum of 20 mils in thickness was cleaned in a soap solution, rinsed, and then passed into a brightening bath for about 3 minutes. The brightening bath consisted primarily of a concentrated, approximately 3 to 1, mixture of sulfuric and phosphoric acids maintained at around 245 C. After thorough rinsing in cold water, the sheet was anodized for about 20 minutes in a 15% sulfuric acid bath with the electrolyzing current being maintained at about 12 amperes per square foot and 15 volts. An oxide coating of approximately 0.2 mil in thickness was obtained. The sheet was thereafter carefully rinsed to free it of any acid and dried to prepare it for receiving an etching composition. The etching composition was provided as a paste comprising about 0.5% by weight of hydrofluoric acid, about 4% by weight of hydrochloric acid, 10% by weight of tragaeanth gum, and the remainder as water. A thin coating of the composition was rolled onto the sheet by a rubber roll having a surface relieved to provide an embossed or raised surface of the pattern desired for reproduction onto the sheet. The etching composition was applied to the raised surface of the roll as a thin film. The composition was thereupon transferred to the sheet simply by rolling contact with the roll. The composition was allowed to remain on the sheet for approximately 1 minute after which it was removed by a spray-rinse of plain water. The partly etched sheet was then placed in a dye hath containing about 2 grams per liter of alizarin red for 1 minute. The dye bath had a temperature of approximately F. After 1 minute the sheet was removed and rinsed to remove excess dye matter and placed in a preliminary aqueous sealing bath of /2% nickel acetate for 2 minutes. The sealing treatment was completed by placing the sheet in a hot water bath (200 to 205 F.) for 20 minutes. The resulting permanently sealed and colored sample sheet was characterized by an aluminum oxide coating dyed to two different shades of red of which the etched areas had the darker shade. The entire dyed coating had a high gloss with the darker red area having apparently as much gloss as the lighter red area.

FIG. 2 FLOW DIAGRAM The FIG. 2 flow diagram is identical to that of FIG. 1 except the etching and anodizing steps are reversed so that anodizing in the how diagram of FIG. 2 is performed where etching was in FIG. 1 while the etching of FIG. 2 is performed where the anodizing step of FIG. 1 was performed. Otherwise there is relatively little difference between the two processes. The etching composition may be made stronger in the second case than it was in the first because the etchant is applied in the second case to bare metal. By selectively etching the bare metal at selective areas, its surface smoothness can be substantially modified. Since the anodic film substantially reproduces the base surface in which it is formed, it will therefore have smooth surface areas corresponding to the smooth portions of the base and rough surface areas corresponding to the etched portion of the base. T o illustrate the process of FIG. 2, Example II is hereinafter given.

EXAMPLE II A sample sheet of aluminum having a thickness of 20 mils cleaned and brightened as described in Example I. After being dried, selected areas of the sheet were coated with a layer of etching composition applied by brush through cut-out areas of a stencil. The etching composition comprised by weight about 7 parts of hydrofluoric acid, 7 parts of hydrochloric acid, 10 parts of tragacanth gum, and the remainder as water. Because of being applied to bare metal, the etching solution herein applied was made substantially stronger than that used in Example I. The etched composition was left on the aluminum sheet for 1 minute and then thoroughly rinsed from the sheet with cold water until all traces of acid were removed therefrom. Thereafter, the sheet was subjected to anodizing to form an aluminum oxide coating as described in Example I. The uncolored oxide coating plainly showed two shades of the natural silver color of aluminum oxide coating. With the sample rinsed free of anodizing solution, it was then placed in a hot bath of ferric ammonium oxalate for 2 /2 minutes, after which the sheet was removed and rinsed with cold water. The

resulting gold colored sheet was sealed as described in Example I. The sheet was characterized by two tones of gold color of highly glossy appearance of which the etched areas had the darker shade.

From the foregoing, it is readily apparent that the coloring process disclosed herein may be conveniently combined with a conventional process which includes an anodizing or other chemical treatment of a metal article resulting in a micro-porous coating thereover to provide a new process by which to obtain a highly decorative multi-color pattern. The present process permits the formation of such patterns with a high degree of complexity, fineness of line, and sharpness of boundary. The process is particularly advantageous when its practice is based on the use of water solutions or dispersions, and water-soluble or water-dispersable reagents whereby Water may be employed as a rinse after each chemical treatment. While described with respect to aluminum, the process in its general aspects is applicable to other metals such as magnesium, tantalum and titanium.

Having described our invention, we claim:

A method of coloring :a metal surface comprising: providing a selected area of said surface with an unsealed micro-porous anodic film of modified adsorptive ability and another area of said surface with an unsealed microporous anodic film of relatively unmodified adsorptive ability; dyeing the modified and relatively unmodified areas with a single selected dye in a single step; and sealin'g said areas; said providing step including a chemical etching treatment in said selected area and a single anodizing treatment for forming said film in both of said areas; said chemical etching treatment being effected by separately etching portions, of said selected area, in different degrees and then rinsing said area in a single rinsing treatment.

References Cited in the file of this patent UNITED STATES PATENTS 2,324,106 Pettit July 13, 1943 FOREIGN PATENTS 468,685 Great Britain July 6, 1937 483,776 Great Britain Apr. 26, 1938 Nov. 10', 1944, pages 

